1. Field of the Invention
The invention relates to a satellite telecommunication system.
2. Description of the prior art
Satellite telecommunication systems are expanding fast because they require an infrastructure which is less costly than that of cable or radio communication systems, in particular for transmitting calls over long distances.
A satellite telecommunication system, for example a geosynchronous system, includes, on board the spacecraft, a transponder including antenna systems and repeater means which receive incoming calls with particular resources, for example a particular frequency and a particular polarization, and transmit them, using other resources, to the destination specified by the call, in order to avoid interference between transmission and reception. In other words, a transmission is allocated a channel which is made up of two frequency/polarization pairs, for example, namely a given frequency and polarization pair for the call reaching the satellite (uplink) and a different frequency/polarization pair for the call retransmitted from the satellite (downlink).
Reducing the energy consumption of a satellite telecommunication system, for the same call capacity, improves its performance commensurately. Also, it is preferable for the resources on board the satellite to be of relatively simple and rugged design in order to minimize the risk of equipment failures.
Satellite transmission systems known in the art until now are of two types, namely a type providing global coverage of a region and a type covering a region by means of areas also known as spots.
In systems of the first type the antenna systems of the satellite have a transmit-receive diagram that covers the whole of the region concerned, which includes a number of urban areas, for example. A connection between two stations or two users is therefore relatively easy to effect because they are in the same antenna coverage area. However, a large coverage does not provide optimum performance. For the uplink, a large coverage area gives a low figure of merit (GT), which imposes the use of large antennas on the ground. For the downlink, a large coverage area requires a relatively high power consumption to provide the receiving station on the ground with a sufficient power density per unit surface area. Also, at any given time, each frequency can be used for only two transmissions, one with a given polarization and the other with the crossed polarization. In other words, a system of the above kind is very greedy in terms of bandwidth.
In the second type of system, the antenna system divides the region into a plurality of areas, each of which corresponds to a country or to a part of a country, for example. In this case, each area is allocated an antenna system whose performance is better than that of the antenna system of the first type of system. For transmissions or calls from one area to another, it is necessary to transmit the calls from one antenna system to another on board the satellite. This transfer from one antenna system to another is effected by permanent wiring or by a switching matrix.
The number of areas is relatively small to limit the wiring or the complexity of the switching matrix, which leads to wide aperture antenna systems whose performance is therefore not always optimum. What is more, permanent connections between antenna systems cannot adapt to traffic changes that may occur in the region concerned. If the traffic in an area increases significantly after the satellite is launched, the incoming call resources of the antenna system corresponding to that area cannot be modified, and because those resources were designed for a particular level of traffic, congestion can cause poor operation for calls uplinked from that area or downlinked to that area.
One way to remedy this drawback that is known in the art is to use dynamic switching matrices on board the satellite to connect the antenna systems corresponding to the various areas, so that the paths and resources in the matrix correspond to the traffic demand at all times. However, this type of dynamic matrix implies complex management and complicated synchronization, in particular between the ground and the satellite.
Nevertheless, compared to the global coverage type of system, the type of telecommunication system with a plurality of areas has the advantage of enabling the same frequency and polarization resources to be used for more than one area, provided that the areas are sufficiently far apart. The same frequency and the same polarization can be used for two far apart areas, this spatial discrimination providing the discrimination between the two calls using the same resources.
The invention relates to a telecommunication system of the second type, i.e. one in which each region is divided into areas. It allows for variations in traffic from one area to another without modification of the resources of the system on board the satellite. It also reduces the complexity of the connecting means between the antenna systems on board the satellite.
The telecommunication system according to the invention covers a region including non-contiguous densely populated areas and is characterized by combining the isolated areas on board the satellite into a plurality of groups each of which uses all of the communication resources allocated to the whole region.
The communication resources include, apart from the frequency bands of the carriers, the polarization, and the transmission times if multiple access techniques are used, for example the frequency division multiple access (FDMA), time division multiple access (TDMA) or code division multiple access (CDMA) technique. If the resources available are the frequency, the polarization and a time slot, at any time a single call or a single packet of a call can use the triplet comprising the frequency, polarization and time slot values. In contrast, another packet (or another call) can at the same time use, for example, the same frequency, a different polarization and the same time slot.
From the point of view of allocating resources, each group of areas is treated like an area in a conventional system.
Thus routing from one area to another on board the satellite is effected between groups and not between areas, which significantly simplifies implementation because the number of connections can be significantly smaller.
The connection between groups is preferably hardwired. Hardwiring is the simplest and most reliable implementation.
Simplicity and reliability also result from the fact that the number of connections is smaller than in prior art systems.
In one preferred embodiment of the invention the allocation of the areas in the groups is such that the traffic in the various groups is substantially equal.
Equalizing traffics between groups is preferable because each group is allocated all of the resources. Thus if an area in a group corresponds to a high level of traffic, that area is the only one in its group or is associated with low-traffic areas.
In other words, the areas to be grouped are chosen to equalize the traffic between the groups.
If the traffic increases in an area, it can be allocated a larger frequency band and/or time allocation and the area(s) in which the traffic has fallen can be allocated a smaller frequency band and/or time allocation.
To take account of traffic changes in the region, it is also possible to provide means for reconfiguring the groups, i.e. means for transferring an area initially allocated to one group to another group. This reconfiguration can be effected by remote-controlled switching means.
Note that the areas of a group can have any geographical distribution. It is not indispensable for them to be adjacent or close together.